Talk:Cross-ratio/Archive 1

Proposed external link
Would the following be an appropriate external link? http://www.ams.org/notices/200810/tx081001234p.pdf Ishboyfay (talk) 20:29, 27 May 2009 (UTC)


 * I think so. The name of the author, the article, and the periodical, and the page numbers and date of publication, should also appear in the article in a list of references. Michael Hardy (talk) 21:56, 27 May 2009 (UTC)

Proposed removal extrenal link
I intend to remove the link to natural gemometry added by Klaus TH. Ruthenberg. It leads to his own website that contains no relevant information at all. Jbogaarts (talk) 10:54, 2 August 2009 (UTC)

Function
Readers looking to find out what cross-ratio may not know what an invariant is, so defining it as an invariant is not helpful. I have suggested that the cross-ratio is a certain function (mathematics) since this term may be more familiar to the reader. As for history, the record will show that references for Pappus have been posted by me. Further, editors should be aware that operating a WP:sockpuppet is not proper. By conducting discussion here in Talk, any differences should work toward resolution.Rgdboer (talk) 00:07, 7 December 2010 (UTC)


 * the Riemann sphere is a projective line, so it may not be necessary to emphasize this in the introduction. Meanwhile, one could mention that one can define the invariant for a 4-tuple on a conic. Tkuvho (talk) 06:07, 7 December 2010 (UTC)

The phrase "special number" now in use has some advantage of clarity to make the article readable. Today I added a link to the Function article, though I see it is weak on functions of several variables (it has section "Functions with multiple inputs and outputs"). Another concern is use of the requirement of collinear points. In the complex plane the cross-ratio exists for four points even if they are not collinear. To say that they are on the complex projective line is sophistry, and contrary to WP:Manual of Style, avoid vague or unnecessarily complex wording.Rgdboer (talk) 02:19, 14 December 2010 (UTC)


 * I am sorry but I completely disagree with this last point. It is not sophistry to say that the complex projective line is a projective line.  Similarly, the cross ratio can be defined for a variety of finite projective planes.  There also the hypothesis is that the four points should be collinear.  Nobody would say it is sophistry to describe a finite collection of points as a "line" in that context.  Once we are on the subject of collinearity, it may be worth mentioning that the invariant can be defined for a (non-collinear) 4-tuple on a conic. Tkuvho (talk) 05:41, 14 December 2010 (UTC)

OK, two favorable facts supporting the necessity of collinearity. Still, say for a student getting some experience with complex variables for the first time, and not having any projective introduction, this requirement may be confusing. Especially when learning that real values of the cross-ratio correspond to four points on a line or circle. Such a student is still thinking of a line in the plane. The outcome of his lessons will be an understanding of the complex projective line (if the instructor has time to cover this notion in the complex variable course). Putting collinearity as required for input values of a cross-ratio does not suit the novice. On the side of finite geometry, in the tradition of Gino Fano's contributions, there is reason to make the stipulation. Given that market demand for the finite science (cryptography) drives a lot of interest, and looking ahead, the subject may adapt from previous development. Perhaps looking at some authors in Complex Analysis will show a path through this collinear ambiguity.Rgdboer (talk) 02:06, 15 December 2010 (UTC)